This invention relates to sample coolers for cooling liquid samples and keeping them cool before they are subjected to an analysis by an apparatus such as a liquid chromatograph for automatically analyzing a liquid sample.
A liquid chromatograph carries out an automatic analysis by mounting vessels preliminarily sealing in small amounts of samples to a rack, setting this rack to an automatic sample injector, causing the automatic sample injector to sequentially suck up the samples from these vessels mounted to the rack and injecting them into the liquid chromatograph according to a specified program. In most situations, the samples on the rack waiting to be analyzed are left under a room temperature condition but there are also situations that some of the samples must be kept at a lower temperature condition in order to prevent decomposition or deterioration. In such a situation, a sample cooler is employed in order to keep these samples under a cooled condition.
Conventional sample coolers are either of the direct cooling type or of the air cooling type. A sample cooler of the direct cooling type uses a rack made of a metallic material with high thermal conductivity and a cooling device such as a Peltier element is attached to the bottom of the rack such that the temperature of the samples can be controlled mainly by heat conduction through solid materials. With a sample cooler of the air cooling type, essential parts of the automatic sample injector including the rack are enclosed inside a heat insulating housing and the air inside the housing is cooled such that the sample temperature is controlled through the air.
Next, the direct cooling type, to which the present invention relates, will be explained more in detail.
FIG. 4 shows one of conventional sample coolers of the direct cooling type. The user initially places liquid samples 4 inside vessels 2 (usually small glass bottles) and closes each of their openings with a septums 3. (Strictly speaking, numeral 3 indicates both a cap and a septum, but they are herein simply referred to as the "septum".) These vessels 2 are mounted onto a rack 1 taken out of an automatic sample injector 7. The rack 1 is made of aluminum and is provided with about 100 holes 5 for accepting these sample-containing vessels 2. Heat (including cold heat) is transmitted to these vessels 2 through the bottoms, as well as the inner surfaces, of these holes 5.
After the sample-containing vessels 2 are mounted to the rack 1, the rack 1 is set on top of a metallic block 23 inside the injector 7. The metallic block 23 is adapted to be cooled by means of a Peltier element 21, serving as a cooling device, attached to its bottom surface, while its upper surface makes a close contact with the bottom of the rack 1 so as to serve as an efficient heat conductor therebetween. It now goes without saying that the rack 1 itself also serves as an efficient heat conductor to the vessels 2. The Peltier element 21 is controlled by a temperature-adjusting device (not shown) to cool the metallic block 23 to a specified temperature by absorbing heat therefrom through its heat-absorbing surface. Heat radiating fins 22 are attached to the back surface (the heat-radiating surface) of the Peltier element 21 on the side facing the interior of an air duct 27 such that the heat transmitted from the metallic block 23 is radiated out and away through these fins 22 and with the aid of an current caused by a fan 28.
The rack 1, the vessels 2 and the liquid samples 4 therein are thus maintained at a specified low-temperature level. The rack 1 is covered with a heat insulating cover 6 in order to be kept at the desired low-temperature level. The top parts of the vessels 2 surrounding their septums 3, however, are exposed from this cover 6 such that samples can be extracted therethrough by means of a sampling needle 13.
The sampling needle 13 is adapted, according to a program, to move freely not only forward, backward, to the left and to the right but also upward and downward by means of a suitable mechanism (not shown), to draw a liquid sample 4 from a vessel 2 by penetrating its septum 3, to transport the drawn liquid sample 4 to the inlet 12 of the liquid chromatograph and to inject the transported liquid sample 4 into the chromatograph so as to have an analysis carried out. Since each analysis by the liquid chromatograph takes tens of minutes, some of the liquid samples 4 mounted to the rack I may have to wait for tens of hours before they are analyzed. Since the liquid samples 4 are maintained at a desired low-temperature level, however, decomposition and deterioration of the liquid samples can thus be avoided.
Prior art sample coolers of the direct cooling type, as described above, have a high heat conducting efficiency and are capable of lowering the temperature to a desired level within a short period of time. Since the top parts of the vessel 2 are exposed to the air at a room temperature and the rack 1 is cooled from below, as described above, however, a non-uniform temperature distribution is likely to result with the bottom parts of the vessels cooled while their upper parts are warm. Moreover, since the lower parts are cool and hence there is no convection, this non-uniformity of temperature distribution does not disappear with time but continues to remain. If the temperature of a vessel is not uniform, it is likely to cause a non-uniformity in the density of the liquid sample inside. An analysis carried out under such a condition is not trustworthy.